Activation of protein kinase C (PKC) by the active phorbol ester 12-myristate 13-acetate (PMA, 100 nM) or phorbol 12,13 dibutyrate (PDBu, 100 nM) reduced taurine uptake by 80% in oocytes injected with cRNA from and expressing the MDCK cell taurine transporter, pNCT. Inhibition of PKC by calphostin C or staurosporine increased taurine uptake 20% and 400% respectively. This inhibitory effect of PMA on taurine uptake was blocked by calphostin C, a specific inhibitor of protein kinase C. Modulation by PMA mainly affected the apparent affinity (Km) (from 5.6 μM to 18.1μM) with minimal effect on the Vmax (25% decrease) of the transporter. A polyclonal antibody (AbS4) directed against a conserved intracellular segment (S4) of the MDCK taurine transporter enhanced taurine uptake by pNCT cRNA-injected oocytes. The effect of AbS4 was blocked by incubation with the corresponding peptide antigen. Pre-immune IgG and peptide antigen had no effect on taurine transporter activity expressed in oocytes. Modulation appeared to occur through phosphorylation of a consensus PKC site located on segment S4 of the transporter, because down-regulation of the transporter by PMA (100 nM) was abolished by preinjection of AbS4 (12 ng/oocyte). In contrast, down-regulation of the transporter by PMA could not be restored by AbS4 when pNCT-expressing oocytes were pretreated with PMA (50 nM). In conclusion, the peptide segment recognized by this antibody appears to participate directly in taurine transporter inactivation that is modulated by protein kinase C phosphorylation.